Fig. 4. Topical opioid interactions.

Opioid/opioid interactions were assessed using dose-response curves of the drugs alone and using a fixed a fixed ratio of a) morphine/methadone (2:1) or b) morphine/meperidine (3.5:1). The ED₅₀ value of the combination with the 95% confidence limits was determined from the dose-response curve and is presented in Table 1. The data is presented as an Isobologram in which only ED₅₀ values are shown. The values on the axis represent the ED₅₀ values of the drugs alone. The line connecting these two points represents the line of addivitity. The combination of morphine and methadone revealed no overlap of the 95% confidence limits, implying a statistically significant interaction. The combination of morphine and meperidine did show overlap and thus did not reach statistical significance.